Definition of Waste Gas Storage Tank Radioactivity Limits
See the memorandum from J. S. Bland to J. P. Stohr dated August 28, 1980, and the incoming request from J. P. Stohr dated July 2, 1980. The wording "equivalent Xe-133" and "considered as Xe-133" in Standard Technical Specifications allow the licensee to use area radiation monitoring readings coupled with a calculational method to approximate inventories in waste gas delay tanks (WGDT).
NUREG-0472, "Radiological Effluent Technical Specifications for PWR's," Section 184.108.40.206 limits the amount of radioactivity in each waste gas storage tank to (x) curies of noble gas. Section 220.127.116.11 further states that the activity shall "be considered as Xe-133." However, the document fails to provide a definition of "considered as Xe-133" or provide a definition of how this determination is to be made. There is also inconsistent wording between NUREG-0472 which presents a "considered as Xe-133" limit and the STS Guidance Document (NUREG-0133) which describes the limit as "Xe-133 equivalent."
The wordings "Xe-133 equivalent" and "considered as Xe-133" were included for the purpose of identifying to licensees the applicable use of area radiation monitor readings in determining an approximate tank radioactivity inventory. The intent of the STS requirement was not to require daily isotopic analysis of the WGDT inventories. Instead, the licensee is allowed to use area radiation monitor readings coupled with a calculational method to approximate tank inventories. Realizing that isotopic distributions change with increased storage times, licensees must demonstrate the applicability of any calculational method employed for this purpose.
In determining the curie limits during licensing, NRR evaluates the expected radionuclide distribution and conservatively establishes a limit such that under accident conditions (decay tank rupture) offsite dose will be less than 0.5 rem. The limit, as presented, is a cumulative sum of the total radionuclide distribution evaluated during licensing. Therefore, considering the inventory limit as a gross activity limit is consistent with the formulation of the "considered as Xe-133" limit and the STS basis which describes the limit as "Xe-133 equivalent."
Regulatory references: Technical Specifications
Subject codes: 7.3, 9.1